Apparatus and method for depositing and reflowing solder paste on a microelectronic workpiece

ABSTRACT

Stenciling machines and methods for forming solder balls on microelectronic workpieces are disclosed herein. In one embodiment, a method for depositing and reflowing solder paste on a microelectronic workpiece having a plurality of dies includes positioning a stencil having a plurality of apertures at least proximate to the workpiece. The method further includes placing discrete masses of solder paste into the apertures and reflowing the discrete masses of solder paste while the stencil is positioned at least proximate to the workpiece and while the discrete masses are in the apertures. In another embodiment of the invention, a stenciling machine for depositing and reflowing solder paste on the microelectronic workpiece includes a heater for reflowing the solder paste, a stencil having a plurality of apertures, and a controller operatively coupled to the heater and the stencil. The controller has a computer-readable medium containing instructions to perform the above-mentioned method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/012,584 filed Dec. 14, 2004, now U.S. Pat. No. 7,347,348, which is adivisional of U.S. patent application Ser. No. 10/226,509 filed Aug. 22,2002, now U.S. Pat. No. 6,845,901, both of which are incorporated hereinby reference in their entireties.

TECHNICAL FIELD

The present invention relates to an apparatus and method for depositingand reflowing solder paste on a microelectronic workpiece.

BACKGROUND

Microelectronic devices are used in cell phones, pagers, personaldigital assistants, computers and many other products. A packagedmicroelectronic device can include a microelectronic die, an interposersubstrate or lead frame attached to the die, and a molded casing aroundthe die. The microelectronic die generally has an integrated circuit anda plurality of bond-pads coupled to the integrated circuit. Thebond-pads are coupled to terminals on the interposer substrate or leadframe. The interposer substrate can also include ball-pads coupled tothe terminals by traces in a dielectric material. An array of solderballs is configured so that each solder ball contacts a correspondingball-pad to define a “ball-grid” array. Packaged microelectronic deviceswith ball-grid arrays generally have lower profiles and higher pincounts than conventional chip packages that use a lead frame.

Packaged microelectronic devices are typically made by (a) forming aplurality of dies on a semiconductor wafer, (b) cutting the wafer tosingulate the dies, (c) attaching individual dies to an interposersubstrate, (d) wire-bonding the bond-pads to the terminals of theinterposer substrate, and (e) encapsulating the dies with a moldingcompound. It is time consuming and expensive to mount individual dies tointerposer substrates. Also it is time consuming and expensive towire-bond the bond-pads to the interposer substrate and then encapsulatethe individual dies. Therefore, packaging processes have became asignificant factor in producing semiconductor and other microelectronicdevices.

Another process for packaging devices is wafer-level packaging. Inwafer-level packaging, a plurality of dies is formed on a wafer and thena redistribution layer is formed on top of the dies. The redistributionlayer has a dielectric layer, a plurality of ball-pad arrays on thedielectric layer, and traces coupled to individual ball-pads of theball-pad arrays. Each ball-pad array is arranged over a correspondingdie, and the ball-pads in each array are coupled to correspondingbond-pads on a die by the traces in the redistribution layer. Afterforming the redistribution layer on the wafer, a highly accuratestenciling machine deposits discrete blocks of solder paste onto theball-pads of the redistribution layer to form solder balls.

The stenciling machine generally has a stencil and a wiper mechanism.The stencil has a plurality of holes configured in a patterncorresponding to the ball-pads on the redistribution layer. The wipermechanism has a wiper blade attached to a movable wiper head that movesthe wiper blade across the top surface of the stencil. In operation, avolume of solder paste is placed on top of the stencil along one side ofthe pattern of holes. A first microelectronic workpiece is then pressedagainst the bottom of the stencil and the wiper blade is moved acrossthe stencil to drive the solder paste through the holes and onto thefirst microelectronic workpiece. The solder paste deposited on themicroelectronic workpiece forms small solder paste bricks on eachball-pad. The first microelectronic workpiece is then removed from thebottom of the stencil, and the process is repeated for othermicroelectronic workpieces that have the same pattern of ball-pads.

After forming the solder paste bricks on the ball-pads, themicroelectronic workpiece is transferred to a reflow oven. The entiremicroelectronic workpiece is heated in the oven to reflow the solder(i.e., to vaporize the flux and form solder balls from the solder pastebricks). The reflow process creates both a mechanical and electricalconnection between each solder ball and the corresponding ball-pad afterthe reflowed solder has cooled and solidified.

Conventional solder printing equipment and processes, however, haveseveral drawbacks. For example, after the microelectronic workpiece isremoved from the stencil, residual solder paste may remain in the holesof the stencil. The residual solder paste can cause inconsistencies inthe size and shape of the deposited solder paste bricks. For example,when the process is repeated with residual solder paste in the holes, aninsufficient volume of solder paste may be placed onto the ball-pads ofthe subsequent microelectronic workpiece. This may create solder ballsthat are too small for attachment to another device. Additionally, thevolume of the residual solder paste may vary across the stencil. Thisresults in different sizes of solder paste bricks across the workpiece,which produces different sizes of solder balls.

Another drawback of conventional processes is that solder paste can besmeared while the microelectronic workpiece is moved from the stencilingmachine to the reflow oven. Even if the solder paste is not smeared,when the pitch between the solder paste bricks is small, the solderpaste on several ball-pads may bridge together after the microelectronicworkpiece is removed from the stencil. Accordingly, a new stencilingmachine and a new method for applying solder paste to microelectronicworkpieces is needed to improve wafer level packaging processes.

SUMMARY

The present invention is directed to stenciling machines and methods forforming solder balls on microelectronic workpieces. One aspect of theinvention is directed to a method for depositing and reflowing solderpaste on a microelectronic workpiece having a plurality ofmicroelectronic dies. In one embodiment, the method includes positioninga stencil having a plurality of apertures at least proximate to theworkpiece and placing discrete masses of solder paste into theapertures. The method further includes reflowing the discrete masses ofsolder paste while the stencil is positioned at least proximate to theworkpiece and while the discrete masses are in the apertures. In oneaspect of this embodiment, the discrete masses of solder paste can beplaced into the apertures and proximate to bond-pads of the dies orball-pads in or on a redistribution layer of the microelectronicworkpiece. In a further aspect of this embodiment, reflowing the solderpaste can include heating the solder paste with infrared light, a laser,a gas, or another device to reflow the solder paste. The heating devicecan be movable relative to the stencil or stationary, such as a heatingdevice having heating elements in the stencil or in a microelectronicworkpiece holder.

In another embodiment of the invention, a method for forming solderballs on the microelectronic workpiece includes placing solder pasteinto the plurality of apertures in the stencil. The apertures in thestencil are aligned with corresponding ball-pads or bond-pads of themicroelectronic workpiece. The method further includes forming solderballs within the apertures and on the ball-pads or bond-pads. In afurther aspect of this embodiment, forming solder balls can includeheating the solder paste in the apertures through convection. In anotheraspect of this embodiment, placing solder paste can include wipingsolder paste across the stencil in a first direction to press discreteportions of the solder paste into the apertures. In a further aspect ofthis embodiment, the method can also include separating themicroelectronic workpiece from the stencil after forming the solderballs.

Another aspect of the invention is directed to a stenciling machine fordepositing and reflowing solder paste on the microelectronic workpiece.In one embodiment, the stenciling machine includes a heater forreflowing the solder paste, a stencil having a plurality of apertures,and a controller operatively coupled to the heater and the stencil. Thecontroller has a computer-readable medium containing instructions toperform any one of the above-mentioned methods. In one aspect of thisembodiment, the heater can include an infrared light source, a lasersource, or a gas source. In another aspect of this embodiment, theheater can be movable relative to the stencil, such as movable laterallyover the top surface of the stencil. Moreover, the heater can includeelements that are stationary, such as heating elements that arepositioned in the workpiece holder or in the stencil. In another aspectof this embodiment, the machine can also include a wiper to force solderpaste into the apertures in the stencil.

In another embodiment, a stenciling machine includes a stencil having aplurality of holes and a moveable wiper configured to move a mass ofsolder paste across the stencil. The moveable wiper is also configuredto press discrete portions of the mass of solder paste into the holesand onto the microelectronic workpiece. The machine further includes aheating means for reflowing the discrete portions of solder paste in theplurality holes and on the microelectronic workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional view of a stenciling machinedepositing solder paste onto a microelectronic workpiece in accordancewith one embodiment of the invention.

FIG. 1B is a schematic cross-sectional view of the stenciling machine ofFIG. 1A having a heat source in accordance with one embodiment of theinvention.

FIG. 1C is a schematic cross-sectional view of the microelectronicworkpiece including the attached solder balls after removing thestencil.

FIG. 2 is a schematic cross-sectional view of a stenciling machinehaving a heat source in accordance with another embodiment of theinvention.

FIG. 3 is a schematic cross-sectional view of a stenciling machinehaving a heat source in accordance with yet another embodiment of theinvention.

FIG. 4 is a schematic cross-sectional view of a stenciling machinedepositing solder paste onto a microelectronic workpiece in accordancewith another embodiment of the invention.

FIG. 5 is a schematic view of a stenciling machine in accordance withanother embodiment of the invention.

DETAILED DESCRIPTION

The following description is directed toward microelectronic workpiecesand methods for forming solder balls on microelectronic workpieces. Theterm “microelectronic workpiece” is used throughout to includesubstrates upon which and/or in which microelectronic devices,micromechanical devices, data storage elements, and other features arefabricated. For example, microelectronic workpieces can be semiconductorwafers, glass substrates, insulative substrates, or many other types ofsubstrates. Many specific details of several embodiments of theinvention are described below with reference to microelectronicworkpieces having microelectronic dies and in some applicationsredistribution layers to provide a thorough understanding of suchembodiments. Those of ordinary skill in the art will thus understandthat the invention may have other embodiments with additional elementsor without several of the elements described in this section.

A. Environment

FIG. 1A is a schematic cross-sectional view of a stenciling machine 180for depositing solder paste 140 onto a microelectronic workpiece 100 inaccordance with one embodiment of the invention. The microelectronicworkpiece 100 can include a substrate 108 having a plurality ofmicroelectronic devices and a redistribution layer 120 formed on thesubstrate 108. In the illustrated embodiment, the microelectronicdevices are microelectronic dies 110. Each microelectronic die 110 canhave an integrated circuit 111 (shown schematically) and a plurality ofbond-pads 112 coupled to the integrated circuit 111. The redistributionlayer 120 provides an array of ball-pads for coupling the bond-pads 112on the microelectronic die 110 to another type of device such as aprinted circuit board. The redistribution layer 120 has a dielectriclayer 121 with a first surface 126 facing away from the dies 110 and asecond surface 127 adjacent to the dies 110. The redistribution layer120 also has a plurality of ball-pads 122 and a plurality of traces 124in or on the dielectric layer 121. The ball-pads 122 are arranged inball-pad arrays relative to the dies 110 such that each die 110 has acorresponding array of ball-pads 122. The traces 124 couple thebond-pads 112 on the microelectronic dies 110 to corresponding ball-pads122 in the ball-pad arrays.

The stenciling machine 180 in the illustrated embodiment includes astencil 130, a wiper assembly 150, and a controller 102 operativelycoupled to the stencil 130 and the wiper assembly 150. The stencil 130has a plurality of apertures 132 arranged in a pattern to correspond tothe ball-pads 122 on the microelectronic workpiece 100. Morespecifically, each aperture 132 in the stencil 130 is arranged so as toalign with a particular ball-pad 122 in the redistribution layer 120.The stencil 130 also includes a first surface 134, a second surface 136opposite the first surface 134, a first end 137, and a second end 138opposite the first end 137. The stencil 130 has a thickness T from thefirst surface 134 to the second surface 136 that corresponds with adesired thickness of a solder paste brick on each ball-pad. The wiperassembly 150 can include an actuator 152 and a blade 154 coupled to theactuator 152. In the illustrated embodiment, the actuator 152 moves theblade 154 across the stencil 130 from the first end 137 to the secondend 138 to drive a solder paste 140 into the apertures 132. In otherembodiments, other stenciling machines can be used, such as machinesthat use print heads or pins to deposit the solder paste into aperturesin a stencil.

B. Depositing Solder Paste

In operation, the controller 102 moves the microelectronic workpiece 100to press the first surface 126 of the redistribution layer 120 againstthe second surface 136 of the stencil 130. Each aperture 132 in thestencil 130 is positioned over a corresponding ball-pad 122 on themicroelectronic workpiece 100. A large volume of the solder paste 140 ison the first surface 134 at the first end 137 of the stencil 130. Next,the wiper assembly 150 moves across; the first surface 134 of thestencil 130 in a direction D₁ from the first end 137 to the second end138. The wiper blade 154 presses a portion of solder paste 140 into theapertures 132 to form solder paste bricks 142 on the ball-pads 122. Thewiper 154 sweeps the remaining solder paste 140 to the second end 138 ofthe stencil 130.

C. Forming Solder Balls

FIG. 1B is a schematic cross-sectional view of the stenciling machine180 of FIG. 1A having a heat source 290 in accordance with oneembodiment of the invention. The heat source 290 is operatively coupledto the controller 102 to reflow the solder paste 140 in the apertures132 of the stencil 130 before separating the stencil 130 from theworkpiece 100. In the illustrated embodiment, the heat source 290 moveslaterally in the direction D₁ across the stencil 130 over the firstsurface 134 from the first end 137 to the second end 138. As the heatsource 290 moves over each aperture 132, the solder paste 140 isreflowed in the aperture 132. More specifically, the heat source 290heats the solder paste 140, vaporizes the flux, and melts the solder. Inone aspect of this embodiment, the heat source 290 heats the solder toat least approximately 200° C. In other embodiments, the heat source 290heats and melts the solder at a temperature less than 200° C. The moltensolder naturally forms into spherically shaped balls on the ball-pads122 of the microelectronic workpiece 100 because of the surface tensionof the molten solder. After the heat source 290 moves past the apertures132, the molten solder cools and solidifies into solder balls 240. Thewetting characteristics between the molten solder and the ball-pads 122causes the solder balls 240 to form on top of the ball-pads 122 creatinga mechanical and electrical connection between the solder balls 240 andthe ball-pads 122.

In one embodiment, the stencil 130 can be made of a nonwettablematerial, such as Kapton® manufactured by DuPont, so that the moltensolder does not stick to the sidewalls 233 of the apertures 132. Thenon-vetting aspect of the stencil 130 further forces the molten solderinto sphere-like balls or other solder elements on top of the ball-pads122. The particular material for the stencil, therefore, should beselected so that the stencil resists wetting by a liquid state of thesolder material. As such, materials other than Kapton® can be used forthe stencil, such as any material that repels the liquid state of thesolder material.

In other embodiments, the heat source 290 can follow the wiper assembly150 (FIG. 1A) as it moves from the first end 137 of the stencil 130 tothe second end 138, or the heat source 290 can be stationary relative tothe stencil 130. In any of the foregoing embodiments, the heat source290 can be a laser, an infrared light, a radiating element or othersuitable heat sources. In other embodiments, the heat source 290 canheat the solder paste 140 by convection, such as by blowing a hot gasonto the solder paste 140.

FIG. 1C is a schematic cross-sectional view of the microelectronicworkpiece 100 including the attached solder balls 240 after separatingthe workpiece 100 from the stencil 130. After the solder balls 240 areformed on the ball-pads 122 in the reflow process, the microelectronicworkpiece 100 is moved in a direction D₂ and released by the stencil130. Alternatively, the stencil 130 can be raised relative to theworkpiece 100. In either circumstance, the solder-balls 240 remain onthe ball-pads 122 because the cross-sectional dimension of thesolder-balls 240 is less than that of the apertures 132 in the stencil130. The solder-balls 240 are smaller than the apertures 132 because theflux in the solder paste bricks 142 (FIG. 1A) vaporizes during thereflow stage.

One advantage of the illustrated embodiments is that reflowing thesolder paste 140 before disengaging the microelectronic workpiece 100from the stencil 130 eliminates the problems that occur when residualsolder paste remains in the apertures 132 of the stencil 130. In theillustrated embodiments, no residual solder paste remains in the stencil130 after reflow because the stencil 130 repels the molten solder, thereflow process reduces the volume of the solder by vaporizing the flux,and the molten solder naturally forms into the solder elements.Moreover, the solder-balls 240 are typically allowed to harden andadhere to the ball-pads 122 before the microelectronic workpiece 100 isseparated from the stencil 130. As such, neither the solder paste bricks142 nor the solder-balls 240 remain attached to the stencil 130 afterseparating the stencil 130 from the workpiece 100.

Another advantage of the illustrated embodiments; is that solder pastebricks 142 will not be smeared or bridged on the workpiece 100. In theillustrated embodiment, the solder paste 140 is formed into hardenedsolder balls 240 before the microelectronic workpiece 100 is removedfrom the stencil 130. As such, no smearing or bridging occurs on theworkpiece 100. A further advantage of the illustrated embodiments isthat stencil machines and reflow equipment are combined in a singlemachine to reduce the floor space for forming solder balls.

D. Alternate Embodiments

FIG. 2 is a schematic cross-sectional view of a stenciling machine 380having a heat source in accordance with another embodiment of theinvention. The stenciling machine 380 can include the controller 102,the stencil 130, and the wiper assembly 150 described above withreference to FIG. 1A. The stenciling machine 380 of the illustratedembodiment also includes a workpiece holder 382 having a plurality ofheating elements 390. The workpiece holder 382 is operatively coupled tothe controller 102 and configured to secure the microelectronicworkpiece 100 during the deposition and reflow of the solder paste. Theheating elements 390 are positioned in the workpiece holder 382proximate to the microelectronic workpiece 100 to heat and reflow thesolder paste in the apertures 132 of the stencil 130. The heatingelements 390 heat the microelectronic dies 110, which in turn heat theball-pads 122 of the redistribution layer 120. The heat is transferredfrom the ball-pads 122 to the solder paste to reflow the solder pasteand form the solder balls 240. The heating elements 390 can beresistance heaters, heat exchangers, or other devices to heat theworkpiece holder 382.

FIG. 3 is a schematic cross-sectional view of a stenciling machine 480having a heat source in accordance with another embodiment of theinvention. The stenciling machine 480 can include the controller 102 andthe wiper assembly 150 described above with reference to FIG. 1A Thestenciling machine 480 of the illustrated embodiment also includes astencil 430 having a plurality of apertures 132 and a plurality ofheating elements 490 positioned proximate to the apertures 132 to reflowthe solder paste 140. Heat is transferred from the heating elements 490to the solder paste through the sidewalls 233 of the apertures 132 byconduction and convection to reflow the solder paste and form solderballs 240 on the microelectronic workpiece 100.

FIG. 4 is a schematic cross-sectional view of a stenciling machine 580for depositing solder paste 140 onto a microelectronic workpiece 500 inaccordance with another embodiment of the invention. The microelectronicworkpiece 500 can include a substrate 508 having a plurality ofmicroelectronic dies 510 which can be similar to the microelectronicdies 110 described above with reference to FIGS. 1A-3. For example, eachmicroelectronic die 510 can have an integrated circuit 511 (shownschematically) and a plurality of bond-pads 512 electrically coupled tothe integrated circuit 511.

The stenciling machine 580 of the illustrated embodiment can include thecontroller 102, the wiper assembly 150, and the heat source 290described above with reference to FIGS. 1A-1C. In other embodiments,other heat sources can be used, such as those described in FIGS. 2-3.The stenciling machine 580 also includes a stencil 530 having aplurality of apertures 532 arranged in a pattern to correspond to thebond-pads 512 of the microelectronic workpiece 500. In operation, thewiper assembly 150 of the stenciling machine 580 presses a portion ofthe solder paste 140 into the apertures 532 of the stencil 530 to formsolder paste bricks 542 on the bond-pads 512. Next, the heat source 290can move over each aperture 532 to reflow the solder paste bricks 542and form solder balls on the bond-pads 512.

One advantage of the illustrated embodiments is that forming solderballs within the apertures of the stencil allows the microelectronicworkpiece to have a fine pitch between the bond-pads or ball-pads. Afine pitch is permitted because the stencil separates the solder pastebricks on adjacent bond-pads or ball-pads and thus prevents smearing andbridging between the adjacent bricks before and during reflow.Accordingly, the fine pitch between the bond-pads or ball-pads of themicroelectronic workpiece reduces the size of the microelectronicdevices formed from the workpiece.

FIG. 5 is a schematic view of a stenciling machine 680 in accordancewith another embodiment of the invention. The stenciling machine 680includes a housing 682, a stencil 630 in the housing 680, and a heatsource 690 in the housing 680. The stencil 630 and the heat source 690can be similar or identical to any one the stencils 130 and 430 and theheat sources 290, 390 and 490 described above with reference to FIGS.1A-4. For example, the stencil 630 can have a plurality of aperturesarranged to align over the ball-pads of the microelectronic workpiece100, and the heat source 690 can heat and melt the solder paste brickswithin the apertures of the stencil 630 to produce spherically shapedballs on the ball-pads of the microelectronic workpiece 100. In otherembodiments, the solder balls can be formed on the bond-pads of themicroelectronic workpiece 500 described above with reference to FIG. 4.In the illustrated embodiment, the stencil machine 680 also includes aconveyor 650 having a first end 651 and a second end 652 opposite thefirst end 651 to move the microelectronic workpiece 100 within thehousing 682 to and from the stencil 630 and the heat source 690. Inother embodiments of the invention, the housing 682 may not include theconveyor 650.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thespirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A stenciling machine for depositing and reflowing solder paste on a microelectronic workpiece having a plurality of microelectronic dies, the machine comprising: a stencil having a plurality of apertures and a heating element coupled to the stencil; and a controller operatively coupled to the heating element and the stencil, the controller having a computer-readable medium containing instructions to perform a method comprising: positioning the stencil at least proximate to the workpiece; placing discrete masses of solder paste into the plurality of apertures; and activating the heating element and thereby reflowing the discrete masses of solder paste while the stencil is positioned at least proximate to the workpiece and while the discrete masses are in the apertures.
 2. The stenciling machine of claim 1 wherein the heating element is in the stencil.
 3. The stenciling machine of claim 1, further comprising a means for placing solder paste into the plurality of apertures in the stencil.
 4. The stenciling machine of claim 1 wherein the stencil comprises a nonwettable material.
 5. A stenciling machine for depositing and reflowing solder paste on a wafer, the stenciling machine comprising: a stencil having a plurality of apertures and a plurality of heating elements positioned in the stencil at least proximate to the plurality of apertures; and a controller operatively coupled to the heating elements and the stencil, the controller having a computer-readable medium containing instructions to perform a method comprising: depositing solder paste into the plurality of apertures in the stencil and onto ball-pads of the wafer; and activating the heating elements and thereby reflowing the solder paste within the plurality of apertures.
 6. The stenciling machine of claim 5 wherein the stencil is formed from a nonwettable material.
 7. The stenciling machine of claim 5 wherein activating the plurality of heating elements and thereby reflowing the solder paste includes heating the solder paste through sidewall portions of each of the plurality of apertures.
 8. A stenciling machine for depositing and reflowing solder paste on a microelectronic workpiece having a redistribution layer with bond-sites, the stenciling machine comprising: a stencil having a plurality of apertures and a heat source embedded in the stencil; and a controller operatively coupled to the heater and the stencil, the controller having a computer-readable medium containing instructions to perform a method comprising: positioning the stencil in a first position at least proximate to the redistribution layer with each aperture in the plurality of apertures generally aligned with a corresponding bond-site; depositing discrete masses of solder paste onto the bond-sites through the plurality of apertures while the stencil is in the first position; and reflowing the solder paste while the stencil is in the first position.
 9. The stenciling machine of claim 8 wherein the heat source includes one or more heating elements in the stencil positioned at least proximate to the plurality of apertures.
 10. The stenciling machine of claim 8 wherein reflowing the solder paste while the stencil is in the first position includes activating the heat source and thereby heating the solder paste at least partially by conduction through the stencil.
 11. The stenciling machine of claim 8, further comprising a moveable wiper, and wherein the computer readable medium contains further instructions to activate the moveable wiper and thereby move the solder paste across the stencil and press discrete portions of the solder paste into the apertures and onto the bond-sites.
 12. A stenciling machine for depositing and reflowing solder paste on a microelectronic workpiece having a plurality of microelectronic dies and a redistribution layer with a dielectric layer over the dies and bond-sites coupled to the dies, the stenciling machine comprising: a housing; a stencil within the housing movable relative to the microelectronic workpiece between a first position in which the stencil is at least proximate to the redistribution layer of the microelectronic workpiece and a second position in which the stencil is spaced apart from the redistribution layer; and a heat source within the stencil to reflow discrete masses of solder paste on the bond-sites of the microelectronic workpiece when the stencil is in the first position.
 13. The stenciling machine of claim 12 wherein the stencil includes a plurality of holes generally aligned with the bond-sites when the stencil is in the first position, and wherein the heat source includes one or more heating elements positioned in the stencil at least proximate to the plurality of holes.
 14. The stenciling machine of claim 12 wherein the heat source includes a heater at least partially embedded in the stencil. 